Ignition assembly resistant to actuation by radio frequency and electrostatic energies



Aug. 9, 1966 K. G. RUCKER 3,264,988

IGNITION ASSEMBLY RESISTANT TO ACTUATION BY RADIO FREQUENCY AND ELECTROSTATIC ENERGIES Filed March 6. 1964 2 Sheets-Sheet 1 INVENTOR KLAUS G. RUCKER Aug. 9, 1966 RUCKER 3,264,988

IGNITION ASSEMBLY RESISTANT TO ACTUATION BY RADIO FREQUENCY AND ELECTROSTATIC ENERGIES Filed March 6 1964 2 Sheets-Sheet 2 F I G. 2

SKlN-EFFECT LOSSWHM) STIOAOVOHW INVENTOR KLAUS G. RUCKER United States The present invention relates to an electric ignition assembly protected against accidental firing by radio frequency and static electrical energies.

Protection against accidental exposure to radio frequency is desirable for most military electroexplosive devices and is sometimes needed for commercial caps. Electrostatic and lightning discharges are as much a hazard to electroexplosive devices electromagnetic radiation, and all should be obviated. The problem is to render this protection without substantially increasing the base cost of the device.

It has been found that electric initiators can be protected against accidental firing a result of extraneous electricity without modifying normal manufacturing methods, by using electrical conductors having high skineffect loss as the lead or leg wires. In conventional electric initiators, insulated copper Wires generally are used as the electrical conductors. However, copper wire has comparatively low skin-effect loss and therefore offers very little protection against extraneous electricity.

The present invention provides an electric ignition assembly employing as its lead wires electrioai conductors having high skin-effect loss. It further provides an electric initiator which is protected against accidental firing by radio frequency and static electrical energies by such an ignition assembly. The initiator comprises a shell integrally closed at one end and open at the opposite end, and containing in sequence from said closed end (A) A base charge,

(B) A priming charge contiguous to said base charge,

(C) An ignition composition contiguous to said priming charge, and

(D) An electrical ignition assembly comprising a sealing plug closing the open end of the shell, a pair of spaced electrical conductors having high skin-effect loss extending through said plug and into said shell, and a bridgewire joining the terminals of said conductors and contacting said ignition composition.

In order to illustrate the invention and describe it more clearly, reference is now made to the accompanying drawings, in which:

FIGURE 1 is a cross-sectional elevation view of a radio frequency and static protected electric initiator of this invention; and

FIGURE 2 is a graph comparing the skin effect losses of copper wire with those of a leg wire contemplated by this invention.

Referring now to FIGURE 1, 1 design-ates a shell of metal or plastic which has one closed and one open end. In sequence from said closed end, there is placed within shell 1, a base charge 2, a priming charge 3, and a heatsensitive ignition composition 4. Extending into the shell 1 is a pair of insulated electrical conductors 5 possessing high skin-effect loss, e.g., wire composed (exclusive of insulation) of a cross-sectional area of 28% stainless steel clad on 72% copper or a solid magnetic wire, joined at their terminals by an electrical conductor of high resistance, i.e., bn'dgewire 6, which contacts ignition composition 4. The open end of the shell 1 is closed by a sealing plug 7 composed of any suitable solid material, e.g., rubber.

All parts of the ignition assembly and initiator of this atent invention, with the exception of the electrical conductors, are conventional.

The shin-effect loss of a Wire is the increase in its resistance at high frequencies. The protection against accidental firing is quite adequate tor most purposes if the quotient of the wires radio frequency resistance divided by its direct current resistance is at least about 25 at one megacycle. At one megacycle, a 0.0254 inch diameter copper wire has a radio frequency resistance 2.7 times greater than its direct current resistance. By comparison, the resistance of the above lossy stainless steel-copper conductors increases 72 times when the frequency increases from zero to 1 megacycle. Such electrical conductors are therefore 72/2.7-27 times better than pure copper Wire at 1 meg-acycle.

The conductors can be wires composed of any conductive metal core, preferably copper or aluminum, coated with an alloy exhibiting high permeability and resistivity. The higher the product of the alloys permeability and resistivity the greater the skin-effect loss and the protection against firing by radio frequency and static electrical energies. For example, when the alloy is coated on a highly conductive metal core such as copper or aluminum, this product should be about l0 ohm henry to provide an increase in resistance of about 25. The preferred coating consists of, by weight, a chrome (l4- 18% )-iron (8l%) alloy, comprising 28% of the wires cross section. Also, the electrical conductors can be any solid magnetic wire with a high quotient, preferably about 10 (henry ohm meter when its permeability is divided by its resistivity, e.g., a wire composed of 78.5% Ni-21.5% Fe or 79% Ni-5% Mo-16% Fe. Such conductors give an increase in resistance of at least about 25.

FIGURE 2 is a graphical illustration which compares the skin-effect loss of a 4-in.-length of 0.025 in.-diameter copper Wire (A) and a wire of the same diameter and length composed of a cross-sectional area of 28% stainless steel (1418% Cr, 85-81% Fe, no more than 1% impurities) clad on 72% copper (B). From the graph it can be observed that the RE. losses of (B) increase greatly from DC. to one mc. (megacycle), therefore protection is assured in the broadcast band and improves even more at higher frequencies.

The following examples illustrate the invention:

EXAMPLE 1 Table I shows the radio frequency loss resistances of different lengths of leg wires having the cross-sectional composition described above in connection with FIG- URE 2 and represented in the table by B. The indicated R.F. resistances are based on Q-meter measurements.

Table I Length of Cap D.C. Resist- RF. Resist- RF. Resist- Leg Wire ance with 22 auce of B ance of 13 (ft) gage B wire at 1 mc. at 5 me.

(ohm) (ohm) (ohm) The resistance of the stainless steel layer is much higher than that of the copper core and can be neglected where direct current flows in the copper. Since 22 gage (0.0254- R. F. energy into cap to fire D. C. energy into cap to fire bridgewire resistance-FR. F. resistance of the leg wires Protection factor:

bridgewire resistance+D. C. resistance of the leg Wires Since energy in each case equals 1 R! and the current which must pass through the bridgewire of a given cap to cause initiation is a constant, it is readily seen that the protection factor for an initiator is equal to (the R.F. resistance of the leg wires plus the bridgewire resistance) divided by (the D.C. resistance of the leg wires plus the bridgewire resistance). Consequently, for a conventional cap having a 1 ohm bridgewire and 6 ft. leg wires (B), the protection factor at 1 megacycle is 20.1/ 1.265 (see Table I), which equals approximately 16. Thus,- at 1 mc., 16 times more energy than the actual bridge firing energy must be fed into a 6 ft. leg cap in order to fire it.

EXAMPLE 2 Table II shows leg-to-leg static tests of pure copper Wire and of the same wire (B) employed in Example 1. The tests were conducted by applying the indicated potential to a capacitor of 800 picofarads and then discharging it to ignite the bead. The normalized protection factor was based on arbitrarily assigning the value of 1 unit to the 6" copper wire which offered almost no protection against extraneous electricity. Increasing the copper wire from 6" to 8 ft. gave only about 15% more pro tection, but when the copper wire was replaced by (B) as shown in Table II, the protection tripled.

Table 11 Potential, Energy to Normalized Material kv. Fire, Protection ws. Factor Ignition bead with 6 copper Wire 14. 4 0. 083 1 Ignition bead with 8 it. copper wire 15. 5 0. 096 1. l6 Ignition head with 8 it. (B) wire 26. 2 0. 254 3. 06

(D) an electrical ignition assembly comprising a sealing plug closing the open end of the shell, a pair of spaced electrical conductors extending through said plug and into said shell, the radio frequency resistance of said conductors at one megacycle being at least about 25 times their direct current resistance, and a bridgewire joining the terminals of said conductors and contacting said ignition composition.

2. An electric initiator of claim 1 wherein said electrical conductors are solid magnetic wires having a quotient of permeability divided by resistivity of at least 10 (henry ohmmeter 3. An electric ignition assembly for explosive initiators, comprising a pair of electrical conductors separated from each other by insulation and connected to each other at one of their ends by a bridgewire, the radio frequency resistance of said conductors at one megacycle being at least about 25 times their direct current resistance.

4. An ignition assembly of claim 3 wherein the conductors are composed of a cross-sectional area of 28% stainless steel clad on 72% copper, said stainless steel consisting essentially of 14-18% chromium and -81% iron.

5. An electric initiator protected against accidental firing by radio frequency and static electrical energies, comprising a shell integrally closed at one end and open at the opposite end, and containing in sequence from said closed end (A) a base charge,

(B) a priming charge contiguous to said base charge,

(C) a heat-sensitive ignition composition contiguous to said priming charge, and

(D) an electric ignition assembly comprising a sealing plug closing the openend of the shell, a pair of spaced electrical conductors extending through the plug and into the shell, and a bridgewire joining the terminals of the electrical conductors and contacting the ignition composition, said conductors being wires composed of a conductive metal core of the group consisting of copper and aluminum coated with an alloy having a product of permeability and resistivity of at least 10- ohm henry.

6. An electric initiator of claim 5 wherein said electrical conductors are wires composed of a cross-sectional area of 28% stainless steel clad on 72% copper, said stainless steel consisting essentiaily of 14-18% chromium and 85-81% iron.

OTHER REFERENCES Electric Detonators, P. B. Tweed, Ordnance, vol. 14, No. 238, JanuaryFebruary 1960, pp 65355.

BENJAMIN A. BORCHELT, Primary Examiner.

R. V. LOTTMANN, Assistant Examiner. 

3. AN ELECTRIC IGNITION ASSEMBLY FOR EXPLOSIVE INITIATORS, COMPRISING A PAIR OF ELECTRICAL CONDUCTORS SEPARATED FROM EACH OTHER BY INSULATION AND CONNECTED TO EACH OTHER AT ONE OF THEIR ENDS BY A BRIDGE WIRE, THE RADIO FREQUENCY RESISTANCE OF SAID CONDUCTORS AT ONE MEGACYCLE BEING AT LEAST ABOUT 25 TIMES THEIR DIRECT CURRENT RESISTANCE. 